A sample to be examined in HPLC must be fed into a high-pressure liquid stream, the latter being interrupted for as short a time as possible. High-pressure injection valves that allow a nearly interruption-free switching of the liquid stream are used for this purpose. Such a structure is described in U.S. Pat. No. 3,530,721, for example; the original application for the latter originates from the year 1965.
The refinement of such an injection valve is mentioned, for instance, in U.S. Pat. No. 4,242,909. The basic principle of the valve shown there has largely established itself since then in HPLC. Since the present invention is based on this type of valve, the principle will be described in detail below.
FIG. 1 shows such a high-pressure valve according to prior art in a schematic representation. It consists of a stator 1 and a rotor 2. The stator has a total of six input and output ports 11, 12, 13, 14, 15, 16. The injection valve can be connected via these ports to other functional elements of the HPLC system by means of capillary connections. For the sake of clarity, the necessary high-pressure threaded connectors are not shown in FIG. 1. The ports are formed inside the valve as bores leading to the other side 10 of stator 1. In practically realized valves, differing from the simplified representation in the drawing, the hole circle diameter on the side of the high-pressure connectors is usually larger than on the other side 10. The rotor has a number of arc-shaped grooves 21, 23, 25, which are aligned precisely with the bores of the input and output ports. This is indicated in FIG. 1 by dotted lines. For clearer representation, the rotor is drawn in FIG. 1 with a spacing from the stator. In the assembled state of the valve, this spacing is basically nil; therefore, the surface 20 of rotor 2 rests directly on the surface 10 of stator 1, as is shown in FIG. 2.
FIG. 2 shows the assembled valve from prior art ready for operation. The rotor is pressed against the stator with a pressure force that is indicated by the arrow F, so that a common contact surface is formed between rotor 2 and stator 1, where the two parts seal together. The pressure force F is dimensioned such the arrangement is still sealed even at the highest pressures to be expected.
In the switching position of the valve shown in FIG. 1 and FIG. 2, grooves 21, 23, 25 are oriented with respect to input and output ports 11-16 such that they produce three connections between adjacent input and output ports; specifically, port 11 is connected via groove 21 to port 16, port 13 to 12 and port 15 to 14. Due to the sealing effect at contact surface 301, liquid supplied via port 11, for example, can exit only at port 12.
To switch the valve to a second position, the rotor can be rotated 60 degrees relative to the stator so that the grooves now connect those ports that did not have a connection previously. The direction of rotation is indicated in FIG. 1 by an arrow on the rotor.
The switching is performed by a motorized drive that can rotate rotor 2 with respect to stator 1. The drive was omitted in the drawing for the sake of clarity.
FIG. 3 shows a high-pressure valve according to prior art in a second switching position. As in FIG. 1, rotor 2 is drawn with a spacing away from the stator in order to achieve better recognizability. In the operation-ready assembled state of the valve, on the other hand, the rotor is pressed onto the stator analogously to FIG. 2.
In this second switching position, the above-mentioned connections are interrupted; instead, port 11 is now connected via groove 21 to port 12, port 13 to port 14 and port 15 to port 16.
The advantage of such valves is that they can be used for very high pressures with a sufficient pressing force. In addition, the bores of ports 11, 12, 13, 14, 15, 16 are arranged such that the ends lie on a circle with a very small radius. The grooves 21, 23, 25 then likewise lie on a circle with a very small radius, so that the dead volumes of the valve can be kept very small. High-pressure injection valves with two switching positions and six ports are generally used in HPLC to feed sample liquid into a liquid stream under high pressure. A common method is the so-called “pulled loop” injection principle. This will be explained schematically and in a simplified manner with reference to FIG. 4 and FIG. 5.
FIG. 4 shows a high-pressure valve according to prior art in a plan view; therefore, stator 1 and rotor 2 are directly one behind the other. The stator is shown as being transparent, so that the position of grooves 21, 23, 25 in the rotor can be recognized. The valve is in a first switching position and the rotational direction of the rotor into a second switching position is indicated by an arrow. The components described below are connected to the valve via capillary tubes, which are shown as thick lines in FIG. 4.
A high-pressure pump 40 that can supply a constant flow rate under high pressure is now connected to port 15. In the switching position of the valve as drawn, this flow reaches port 14 through groove 25, and then reaches a chromatographic column 41. A sample needle 44, which barely dips into a sample container 43, is connected to port 12. Instead of being moved into sample container 43, sample needle 44 can be moved into a waste container to dispose of excess liquid. The waste container is not shown in the drawings since whether sample needle 44 is in the sample container or the waste container is irrelevant to the explanation of the invention. A syringe 42 for drawing sample liquid is connected to port 11. The two remaining ports 13, 16 are externally connected to one another via a sample loop 50. Sample fluid can thereby be drawn from sample container 43 into sample loop 50 with the aid of syringe 42. The switching position of the valve as drawn is referred to as the LOAD position, since the sample material is being loaded into the sample loop. The term “load” will be used for this in the remainder of the description. In order to feed the sample material into the high-pressure liquid stream, the valve is switched over to a second switching position, which is shown in FIG. 5.
FIG. 5 shows the high-pressure valve according to prior art in the same representation as in FIG. 4, but in its second switching position. A possible rotational direction of the rotor back into the first switching position is again indicated by an arrow. Now sample loop 50 is looped into the liquid path between pump 40 and column 41. The sample liquid previously drawn into sample loop 50 is thereby transported with the liquid stream coming from pump 40 into column 41, where the chromatographic separation takes place. Additional components for analysis, which are omitted from FIGS. 4 and 5 for the sake of clarity, are generally connected downstream of the column. The switching position of the valve as drawn is referred to as the INJECT position, since the sample material is being injected into the high-pressure liquid. The term “inject” will be used for this in the remainder of the description.
The injection principle as described is used on a standard basis in HPLC, sometimes in modified form. The basic mode of operation with LOAD and INJECT is always the same, with a great variety of implementations in use. For instance, U.S. Pat. No. 4,939,943 describes an autosampler in which a high-pressure syringe, which simultaneously is part of sample loop 50, is used in place of syringe 42. Sampling needle 44 is a component of the sample there as well. The valves that are used can also differ from the above-described design, e.g., additional ports for additional functions can be present; the arrangement of the grooves can also deviate from the plan shown in the drawings. The invention can be applied accordingly to such different designs of samplers as well.
As further prior art one can mention a special construction of high-pressure valves. Such valves are commercially available from Rheodyne LLC, California, e.g., models 7710 and 9710, and allow nearly interruption-free switching of the pump flow.
The basic principle of such valves from prior art corresponds very closely to FIGS. 1-5, and will be explained with reference to FIG. 4. In addition to the grooves 21, 23, 25 in rotor 2, there is also a single groove in stator 1 that, originating from port 14 in the stator, runs parallel to groove 25, but ends before reaching port 15.
During the switching process from the LOAD position to the INJECT position, the direct connection between ports 15 and 14 remains intact at first. The direct connection is not interrupted until just before groove 25 produces the connection between ports 15 and 16. The same effect can also be obtained by reversing the direction of rotation, with the groove originating from port 15 and ending even before reaching the bore of port 14. This function is referred to by the manufacturer as “make-before-break,” since the existing connection is not interrupted until the new connection is produced.
A similar injection high-pressure valve and an autosampler for HPLC that is realized with it is also described in WO 2006/083776 A2. This publication is concerned with the avoidance of pressure variations that are produced by samplers or switching processes in the high-pressure valves, affect the chromatography column, and can damage or destroy it. As a solution, a high-pressure injection valve is specified that, in addition to the grooves provided in the rotor, has at least one extra groove in the stator that serves to maintain the connection of the two high-pressure ports to which the pump and the column are connected for as long as possible during the switching from LOAD to INJECT. The connection is even maintained when the groove permanently connected to one of the sample loop ports reaches the pump port, so that the initially pressure-free sample loop is simultaneously subjected to the pump pressure. Then the high-pressure ports are cut off, only in the last angle range of the rotor's rotational movement, and the column port is connected to the respective other sample loop port. In this manner, almost no pressure change in the column is generated. The column is nearly always subjected to the pump pressure. It is indicated in this document that when there is a shift from INJECT to LOAD, the sample loop can be depressurized in an optimal manner, by means of a corresponding lengthening of the groove in the rotor or by providing a stator groove formed in the stator, by connecting the port connected to the syringe to the rotor groove being moved towards it, and thus to the respective sample loop port, before the other sample loop port is connected to the port connected to the sample needle. While it is possible to achieve a defined decompression of the sample loop due to this advancement, the critical switching states described below, in which a fluid flow into a narrow port can lead to damage to the rotor and/or the stator, arise here as well. In recent years, a trend toward separating columns with a small particle size has been observed in HPLC. Such separating columns allow better separation performance and a faster separation, which is why this is referred to as fast HPLC.
Since the flow resistance increases strongly with decreasing particle size, considerably higher pressures are required for fast HPLC. The maximum column pressure that appears is typically between 100 and 400 bar in conventional HPLC, while 600-700 bar are required for fast HPLC, sometimes even more than 1000 bar. A trend is already beginning to emerge in the direction of columns with even better separation power, which require even higher pressures of up to ca. 2000 bar.
In order to be able to operate high-pressure injection valves at such high pressures, the pressure force F (see FIG. 2) must be correspondingly increased for the valve to maintain integrity. In order for the rotor, which is normally fabricated from plastic for technical and cost reasons, to withstand this force, glass-reinforced or carbon fiber-reinforced plastics are used according to prior art. In addition, there is an increased material stress due to the higher pressure force F and consequently there is excessive wear, so that the service life of the valve (number of switch operations) is unsatisfactory.
This problem can be solved by appropriate material selection or coating. Thus, U.S. Pat. No. 6,453,946 describes a special coating that allows a cost-effective production of rotor and stator and simultaneously sharply reduces the wear on the materials.
It has been shown that such improved valves do behave more favorably, but fail during operation at very high pressures after a relatively small number of switching cycles.
More detailed study of such failed valves from prior art has shown that the failures mainly occur due to material erosion at certain points of the rotor. FIG. 6 shows a photo of such a damaged rotor. Grooves 21, 23, 25 appear shaded due to the side-lighting. The damaged areas 201, 202 are marked by circles and lie in the extension of grooves 23 and 25. In the case of the damage at 201, a deep hole was created in the rotor.
It was additionally found that damage appeared on the stator as well, more particularly, in the vicinity of the bores for the ports. FIG. 7 shows the photo of a such a damaged stator with damaged areas 101, 102.
The origin of such damage can be explained as follows.
In the INJECT position of FIG. 5, sample loop 50 and grooves 23 and 25 are under high pressure. During the changeover process to the LOAD position (FIG. 4), groove 23 remains under high pressure since the sample loop maintains the pressure due to the compressibility of the solvent contained in it. FIG. 8 shows the position of groove 23 in side view, shortly before the end of the changeover from INJECT to LOAD, when the damage in the rotor appears. The curvature of groove 23 is not taken into account in the drawing. Only the area of rotor 2 around groove 23, and only the area of stator 1 around ports 12 and 13, are shown.
The transition from the bores of ports 12, 13 to the sealing surface has, as do all ports 11-16, a respective bevel 121, 131. Sharp edges or burrs that could damage the rotor are thereby avoided. In the position shown in FIG. 8, groove 23 has just reached bevel 121, so that a very small passage has formed between the end of the groove and port 12.
Sample loop 50, and therefore groove 23 as well, are still under almost full pressure, whereas port 12 is connected to sample container 43 or the waste container and therefore has normal air pressure. Thus, the entire pressure difference acts on the very small passage, which simultaneously has only a very short length.
As in a nozzle, this “bottleneck” leads to extraordinarily high flow speeds, and the energy stored in the sample loop due to the compressibility of the solvent is converted into kinetic energy. As in the case of water-jet cutting, a very high energy density arises, which can damage the material in the vicinity.
In FIG. 8, the solvent flowing in from the sample loop is designated by an arrow 61, the solvent flowing out in the direction of the sample container or waste container is designated by an arrow 62, and the flow pattern on bevel 121 is designated by a bundle of arrows 63. The flow that overcomes the bottleneck on the bevel strikes the opposite edge of the bevel at extremely high speed and is deflected there, so that a small amount at this point is virtually washed out. This process is repeated with each switching cycle, so that the damage accumulates, which can lead to the hole 201 shown in FIG. 6.
FIG. 9 shows the flow pattern 63 as in FIG. 8, but in a perpendicular view from above. It is recognizable in this representation that, in striking the opposite edge of bevel 121, the flow 63 is, in a manner, focused by the curved shape onto the damage point, which further amplifies the deleterious effect.
The effect shown in FIG. 8 likewise appears in the switch back from LOAD to INJECT when groove 25 reaches port 16. At this moment, sample loop 50 is depressurized, since it is connected to sample container 43 in the LOAD position. Groove 25, on the other hand, is under the full pressure of the pump. This leads to the damage 202 at the end of groove 25 as shown in FIG. 6. The individual damage patterns differ from one another to some extent, since the associated components (e.g., flow resistance, stored fluidic energy) have an influence.
As is recognizable in FIG. 6, the problem does not appear at the other ends of the grooves, although the same pressure differences between the respective ports are in effect there to some extent. In the changeover from INJECT to LOAD, for instance, depressurized groove 21 reaches port 16, which is connected to sample loop 50, which is under pressure. Nonetheless, no damage occurs at the end of groove 21.
This can be explained with reference to FIG. 10 as follows.
FIG. 10 shows, in the same representation as FIG. 8, the situation shortly before the 11 LOAD position is reached, i.e., before groove 21 reaches port 16. Here the flow 64 enters via port 16, then flows through the bottleneck past bevel 161 into groove 21 and again exits as a flow 65 at port 11. Here too there is a bottleneck at the transition from port 16 to groove 21, at which a high pressure difference is in effect and thus extremely high flow velocities occur. The essential difference from FIG. 8 is that the flow 66 flows through this bottleneck in the opposite direction from the flow 63. The flow thus does not strike the opposite edge of bevel 161 nor any other solid material, but only the fluid present in groove 21. Therefore the flow is braked, or the kinetic energy is reduced, to such an extent that the remaining kinetic energy is too small to cause material damage.
Thus it can be assumed that under otherwise equal conditions a switching process with a reverse flow direction is not harmful. This recognition is used, as described below, for the solution of the problem according to the invention.
In the situation shown in FIG. 10, i.e., the reaching of the LOAD position, however, there is an undesired pressure surge on the syringe 42 connected to port 11 (FIG. 4), since the pressure in sample loop 50 discharges via port 11 at this moment.
The switching directions of the valve shown in FIGS. 4 and 5 are not mandatorily specified according to prior art, instead the switching from the LOAD to the INJECT position can also be done in the opposite rotational direction of the rotor. In this case the damage is avoided at the above-mentioned positions, but identical damage appears at different positions. This can be explained analogously to the consideration above.
The previous discussions explained the damage appearing in rotor 2, but not the damage in stator 1 shown in FIG. 7. This can be explained as follows.
The rotors 2 consist of fiber-reinforced plastics, as already explained. In the manufacturing of grooves 21, 23, 25, it is not possible to completely prevent the ends of the fibers from protruding from the plastic. Such protruding fibers are also found particularly in the vicinity of the edges of the grooves, i.e., in the direct vicinity of the surface of stator 1 in the assembled state of the valve. If, as shown in FIG. 8, the solvent flows from groove 23 to port 12, then the protruding ends of the fibers at the left upper edge of groove 23 are situated precisely where the flow 63 passes through the bottleneck at extremely high speed. The ends of the fibers are thereby pressed into the bottleneck and cause abrasive damage there, particularly because rotor 2 is pressed against stator 1 during the switching of the valve.
This damage also does not occur if the flow direction is reversed as in FIG. 10. In this case, the ends of the protruding fibers are oriented to the right by the flow 66, i.e. in the direction of the center of groove 21, where they cannot cause damage due to abrasion.
For this reason, the damage 101, 102 shown in FIG. 7 occurs at the same ports at which the damage 201, 202 shown in FIG. 6 appears, namely ports 12 and 16.
The damage mechanism as described is only effective if the situation illustrated in FIG. 8 occurs at one of the involved ports. This is the case for only a moment during the switching processes from LOAD to INJECT and vice versa. Therefore, an improvement can be achieved by accelerating this switching process. In this manner, the duration of the critical situation can be shortened and thus the damage can be reduced. Of course, the damage cannot be completely avoided in this manner, but only reduced by precisely the extent to which the switching speed is increased. The leeway for an even higher switching speed is only slight, however, since this would greatly increase the effort and therefore the costs.
Another approach to a solution could be to change the shape of the bevels 121, 131, in particular, to enlarge them so that the flow is directed onto the rotor with less intensity. Here as well, there is only a slight leeway since the bevels in valves according to prior art are optimized such that as little abrasion as possible occurs due to the switching process and the dead volumes are as small as possible. Furthermore, it is at best possible to avoid the damage 201, 202 in the rotor by means of such an approach, but not the damage 101, 102 in the stator.